Scattering scanning near field optical microscopy (s-SNOM) operates by interacting a sharp probe tip of a probe microscope with a sample surface and collecting light scattered from the region of tip-sample interaction. Using this technique it is possible to measure the optical properties of samples with a spatial resolution far below the conventional diffraction limits. The resolution improvement comes from a local enhancement of the incident radiation field due to the sharp tip. The enhanced radiation field interacts with the sample and then scatters radiation into the far field. This near-field enhancement increases the amount of radiation scattered from the tip-sample region such that the scattered radiation can be more easily detected.
Referring to FIG. 1B, a probe 100 with a sharp tip 104 is interacted with a region of interest 106 of a sample 108. Light 110 with electric field intensity Ein is incident on the surface of a sample 108. The incident radiation field is enhanced in the region of the tip apex 104, leading to light scattered from the region of tip-sample interaction with electric field intensity Enf. It is the goal of a s-SNOM system to detect this scattered near field radiation Enf. Unfortunately, the incident radiation Ein also interacts with regions of the probe tip 102 that are away from the tip apex 104 and also with regions of the sample 108 that are away from the tip apex and even away from the region of interest 106. These unwanted interactions result in large background scattering Ebg. In practice, the background scattered field can be orders of magnitude larger than the tip apex scattered field Enf. For this reason it is highly desirable to have effective techniques to discriminate between light scattered from the tip apex region versus scattered from other sources.
Several techniques have been used to attempt to separate the near field light (scattered from the tip apex area) from background scattered light. A commonly used approach is to oscillate the tip, for example using the tip of an atomic force microscope and oscillating it at resonance, such as in tapping mode. Since the amount of near field light scattered from the sample depends strongly on the tip-sample distance, oscillating the tip in and out of contact with the surface modulates the light scattered into the far field. Several approaches have been used to demodulate the tip-scattered light from an oscillating AFM tip. The simplest approach is to use a lock-in amplifier to measure an amplitude of tip-scattered light at the oscillation frequency or a higher harmonic of this oscillation frequency. Stephen Quake also demonstrated the technique of time gating collection of tip scattered photons from fluorescence to correspond with the times that the tip is closest to the sample as described in U.S. Pat. No. 6,953,927.
While each of these approaches has achieved experimental success, each has significant limitations. In the case of simple oscillation of the tip with lock-in detection, the amount of light scattered into the far field depends on both the real and imaginary coefficients of the sample index of refraction and on an unknown arbitrary phase, as well as an unknown and variable amount of background scattering.
Interferometric techniques have also been used to improve detection of tip scattered light. There have been two main approaches, so called “homodyne” approach as described by Taubner et al in Journal of Microscopy, Vol. 210, Pt 3 Jun. 2003, pp. 311-314 and a “pseudoheterodyne” approach as described by Ocelic, Hillenbrand and others, for example in U.S. Pat. No. 7,738,115.
These interferometric techniques are shown generically and schematically in FIG. 1A. Light 122 from a light source 120 is directed through a beam splitter 124 to a sample 126 near the end 128 of probe 130. As indicated in prior description of FIG. 1B, the light incident on the probe and sample, results in light scattered both from the region of interest (Enf) and from background sources (Ebg). This scattered light can be directed back to the beam splitter 124 and then focused on a detector 140. The prior art has also employed well known interferometric techniques by directing a portion 134 of the incident beam at the beam splitter 124 to a reference mirror 136 and then interfering the reference beam with the sample scattered light at the detector 140. A modulator 138 may be used to periodically modulate the reference phase. This interferometric scheme is employed for three main purposes: (1) To provide wavelength sensitive measurements with broadband sources, as commonly performed with Fourier Transform Infrared (FTIR) spectroscopy; (2) to provide amplification for the weak tip-scattered field Enf, as will be explained below; (3) to provide separate measurements of the optical amplitude and phase.
We next consider the signal measured at the detector 140. The total electric field Etot at the detector is given by:Etot=Enf+Ebg+Eref 
where each of these quantities are complex, to account for phase differences between the electric field components. Note that for simplicity, all collection efficiency factors and optical losses are being subsumed into the electric field strengths, i.e., these are the electric field strengths at the detector surface, not at the sources. The light intensity at the detector is proportional to |Etot|2, thus:Id∝(Enf2+Ebg2+Eref2+2EnfEbg+2EnfEref+2EbgEref)
The interferometric scheme in FIG. 1A provides amplification of the near field scattered radiation through the crossterm ErefEnf. Unfortunately, in practice, there have been severe practical limits on amount of this amplification. Worse still, the background scattered light and reference beam light often have similar order of magnitude, and at best have a ratio of Eref:Ebg of ˜3-10 (see for example U.S. Pat. No. 7,738,115, col. 2, lines 64ff). The fact that the reference intensity and background intensity are similar can lead to large errors in measurements of optical phase.
U.S. Pat. No. 7,738,115 describes method of overcoming these errors by separating near field and background fields using a “pseudoheterodyne” technique that uses sinusoidal oscillations of both the probe (130 in FIG. 1) and the reference mirror (138) to isolate the near field term Enf in frequency space. Using narrow band lock-in detection, this technique can obtain optical amplitude and phase measurements for the near field scattered radiation.
There are several disadvantages of the pseudoheterodyne approach, namely (1) loss in signal-to-noise; (2) loss in measurement speed; and (3) increased measurement complexity. The loss in signal-to-noise ratio comes from the fact that the pseudoheterodyne technique distributes the energy from the Enf signal across many frequency bands, specifically numerous sidebands around the cantilever oscillation frequency and its higher harmonics. (See for example FIG. 7 in U.S. Pat. No. 7,738,115 and the illustration in FIG. 3A). Thus demodulating at any single side band samples only a small portion of the original scattered energy. As a result the signal to noise ratio of the measurement is degraded—in the effort to reject background, much of the signal is discarded.
Additionally, the sidebands are very close to the original probe modulation frequency (and its harmonics). Specifically, the sidebands are separated from the cantilever oscillation frequencies by fref, the modulation frequency of the reference arm mirror. The reference arm mirror and associated actuators are relatively large mechanical devices and thus limited in practice to oscillations in the 100's of Hertz range. As such, it is necessary to demodulate the sidebands with a very narrow bandwidth lock-in amplifier, compared to the oscillation frequency of the probe which can be in the megahertz range. The narrow bandwidth required to demodulate the sideband thus slows down the measurement since it requires longer integration times and thus makes the entire measurement much slower.
The current invention overcomes these limitations by providing an optical arrangement that enables Eref>>Ebg. This allows direct demodulation of the scattered near field optical signal with high accuracy measurements of both optical amplitude and phase. Additionally, the technique of the current invention achieves much better signal-to-noise ratio as it can capture a much larger fraction of the signal in fewer and widely spaced frequency bands. This both substantially simplifies and speeds up the demodulation, thus supporting higher speed imaging and spectroscopy.
Another major limitation of prior art s-SNOM systems is the inability to easily calculate a spectrum that closely resembles a traditional infrared absorption spectrum without complicated post-processing involving an in situ reference. The pseudoheterodyne technique can output signals that are proportional to the amplitude and phase of the scattered light. Unfortunately, at each wavelength, the optical phase has an unknown and varying phase offset. Thus the phase versus wavelength (or wavenumber) plot does not closely resemble a conventional absorption spectrum. To convert the phase signal into something approaching an absorption spectrum, it has been necessary to use an in situ reference sample with well-known phase behavior over the wavelength range of interest. The in situ reference sample requirement has led to the need that samples be prepared with an additional known material directly adjacent to the sample of interest. In fact most if not all in situ reference measurements are performed such than a material of interest is sufficiently close to the in situ reference such that the material of interest and the reference material can be imaged in the same field of view of the same AFM image. The in situ reference sample also must have a flat or otherwise well-known phase behavior. This in situ reference requirement has dramatically limited the types of samples that can be successfully measured, as many if not most real world samples do not have a suitable reference material available. Therefore special sample preparation steps are required to prepare a sample with material of interest on a substrate that can serve as a reference material or to prepare the material of interest with an in situ reference sample adjacent to the material of interest.
Further, any errors in either the measured or assumed phase of the in situ reference sample lead directly to errors in the calculated spectrum of an unknown sample. In practice, absorption spectra calculated from prior art s-SNOM measurements have contained distortions in absorption band shapes, offsets in absorption peak positions, and errors in relative absorption peak heights. These errors lead to complications in the interpretation of s-SNOM spectra and discrepancies from the standard spectra known from materials databases. Additional errors may be present in systems such as FIG. 1A, or in other system with similar arrangements. The scattered light from the tip-sample region 110 is interfered with light in the reference arm 134, which may be completely separate from the sample arm of the interferometer 122. This interferometer scheme amplifies the weak tip-scattered light and also enables measurements of the optical phase of the scattered light. The disadvantage of interferometric detection is that it is extremely sensitive to differential changes in the optical path length between the sample arm and reference arm, for example due to temperature or air current fluctuations.
Next we turn to issues of spectroscopic measurements in the prior art using s-SNOM techniques. For many years s-SNOM was primarily an imaging technique, but in recent years it has been possible to collect optical spectra from sub-micron regions of a sample. There have been two basic approaches. In one approach a broadband laser is used to illuminate the sample simultaneously with light from multiple wavelengths. In this case interferometric Fourier transform techniques are used to deconvolve the wavelength dependent scattering to obtain near field spectra. This technique is outlined for example in publications from the Hillenbrand research, for example in Amenabar et al. Nat Commun 4 (2013). This technique requires sophisticated femtosecond lasers which can be expensive, complicated, power limited and have limited spectral coverage.
Alternately, a technique sometimes referred to as “spatio-spectral imaging” is employed. In this case a narrow band tunable source, for example a quantum cascade laser is used to collect a series of s-SNOM images at different wavelengths. To obtain spectra from this technique, however, is usually extremely tedious. For example, each s-SNOM image at each center wavelength may take 5-20 minutes to acquire. To acquire even a minimal spectrum at 10 different wavelengths for example would require 50-200 minutes, just to obtain the images. For a more useful spectrum, for example covering 400 cm−1 with 4 cm−1 spectral resolution would require 101 points thus requiring 505-2020 minutes, or 8-33 hours. As such it has been impractical to use the spatio-spectral technique for rapid point spectroscopy, i.e. measuring the absorption spectrum of a single point. The AFM-IR technique, described in U.S. Pat. No. 8,001,830, by comparison does not require an in-situ reference and point spectra can be obtained in around a minute. Some samples are not suitable for measurement by AFM-IR, however, and it is desirable to have a method of obtaining point spectra using the s-SNOM technique on similar time scales.